Trouble with repeatable DPD/FAS FC test

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jeffbg

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Jul 29, 2008
63
Folks,

I'm having problems getting accurate readings. Here is what I mean. I seem to be off by plus or minus 0.5 or so on a given reading. I did two tests this morning within 5 minutes with no equipment running and one was a FC reading of 4.5 and the other 5.0. As best I can tell, I did everything the same way. I notice that the meniscus on my "chlorine only" cylinder is a bit stronger than the other one for what it's worth, but I try and be careful.

Is this normal for this test? Is there something I could do to make it more repeatable?

-- jeff
 
That's about as good as it gets and is perfectly adequate for most applications I can think of.

If you require more accuracy, use a 25ml sample for an accuracy of .2ppm (+- .2)

Why do you feel you need the additional accuaracy?
 
The test is accurate to within +/- 1 drop, which is all the accuracy you'll ever need for your pool. For the 10ml sample that's +/- .5ppm, and for the 25ml sample it's +/- .2ppm.

If you're asking because you want to verify that your procedure is accurate, then do several tests in quick succession from the same water sample. If your results are within +/- 1 drop you're good to go.
 
jeffbg said:
I did two tests this morning within 5 minutes with no equipment running and one was a FC reading of 4.5 and the other 5.0.
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Not sure why you took your samples with no equipment running (it's very likely, as Beez said, that you'll get better accuracy with just one sample.)

Just the act of taking two samples from the same spot (one right after the other) may give you different results. I like to mix up the water via pumping for at least a few minutes and then take ONE sample from a point furthest from returns/water features. If you have a larger pool (30K+ gallons) consider taking two samples from opposite ends within a minute or two of each other, then check chlorine and pH (first) of both samples; you can average together the results.
 
polyvue said:
jeffbg said:
I did two tests this morning within 5 minutes with no equipment running and one was a FC reading of 4.5 and the other 5.0.
Click to expand...
Not sure why you took your samples with no equipment running (it's very likely, as Beez said, that you'll get better accuracy with just one sample.)

Just the act of taking two samples from the same spot (one right after the other) may give you different results. I like to mix up the water via pumping for at least a few minutes and then take ONE sample from a point furthest from returns/water features. If you have a larger pool (30K+ gallons) consider taking two samples from opposite ends within a minute or two of each other, then check chlorine and pH (first) of both samples; you can average together the results.
Click to expand...

The point of my "no equipment running" was to make the point that since I have a SWG, the variability in my readings was not due to the SWG running. The water features had run already this morning before my reading and had mixed up my spa and pool, so it had been "stirred" since sitting overnight. I try and take my samples away from the returns and the autofill, and approximately the same place every day to get consistency.

Sounds like 0.5ppm is about as accurate as it gets, but makes me wonder if the overnight drop test could be a bit off since if you drop 0.5ppm and have an error of 0.5ppm, you could think you've lost 1ppm, and think you have an algae problem.

-- Jeff
 
jeffbg said:
Sounds like 0.5ppm is about as accurate as it gets, but makes me wonder if the overnight drop test could be a bit off since if you drop 0.5ppm and have an error of 0.5ppm, you could think you've lost 1ppm, and think you have an algae problem.
Click to expand...
The last two lines of the Overnight FC Test Instructions specifically address this; overnight loss of up to 1.0ppm is okay.
--paulr
 
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